The invention relates generally to engines used in vehicles, and more particularly, to a system and method for controlling a dual fuel engine.
In a compression-ignition engine, such as a diesel engine, a fuel injection system injects fuel (e.g. diesel fuel) into compressed air within each of the engine cylinders to create an air-fuel mixture that ignites due to the heat and pressure of compression. Unfortunately, engine efficiency, power output, fuel consumption, exhaust emissions, and other operational characteristics are less than ideal. In addition, conventional techniques to improve one operational characteristic often worsen one or more other operational characteristic. For example, attempts to decrease specific fuel consumption often cause increases in various exhaust emissions. Vehicle exhaust emissions include pollutants such as carbon monoxide, nitrogen oxides (NOx), particulate matter (PM), and unburned hydrocarbons (UHC) generated due to incomplete combustion of fuel within the combustion chamber. The amount of these pollutants varies depending on the fuel-air mixture, compression ratio, injection timing, ambient conditions, and so forth.
In the oil and gas market and transportation sector, for example, the fuel bill is one of the contributors to the total cost of operation. The rapid expansion and abundance of natural gas in some areas of the world is driving a dramatic cost advantage of natural gas over diesel fuel, making natural gas a very economical fuel source. A dual fuel engine is based on a traditional diesel engine, with the addition of dual fuel specific hardware. When the engine is operating in dual fuel mode, natural gas is introduced into an intake system. Near the end of the compression stroke, diesel fuel is then injected. The diesel fuel ignites, and the diesel combustion causes the natural gas to burn. However, for a dual fuel operation, there is a narrow region of air to fuel ratio for which safe operation of the engine can occur at a given operating condition. For instance, if the cylinder contents are too lean (i.e., the air to fuel ratio is too high), there is the potential for misfire, and if the cylinder contents are too rich, (i.e., air to fuel ratio is too low), there is the potential for knock.
There is a need for an enhanced system and method for controlling a dual fuel engine such that the air to fuel ratio is within a desired operating region for each operating condition.